#1) A few tens of thousand base-pairs length of DNA (or RNA) with a layer ofproteins wrapped around its outside (and maybe also a layer of membrane wrapped around that!) Suppose that the proteins have some mechanism to get the DNA into cells of some bacteria, or some animal plant. (usually species specific) Suppose that this DNA gets copied in the host cells; and suppose that the DNA gets transcribed to make m-RNA and the host cell ribosomes and t-RNA are used to make the proteins that the DNA codes for, And that some of these proteins form a layer around the DNA copies, and the copies + proteins get released by the host cells. That would be the life cycle of a typical virus. #2) Many diseases are caused by viruses: examples include: influenza, measles, smallpox, AIDS, colds #3) Viral diseases are not treatable by antibiotics; (but there are anti-viral drugs; analogs to purines & pyrimidines; but unfortunately none of these are very effective) The immune system, and its stimulation by vaccines, is our main defense against viruses. #4) Some viruses use single stranded DNA as their genetic material other viruses use double stranded DNA, Other viruses use RNA, in some kinds single-stranded and in other kinds double-stranded RNA. In one group of viruses, the original virus has RNA, but also produces enzymes that make DNA with the same base sequence. "Reverse transcriptase" is the name of this enzyme. The AIDS virus does this. #5) All viruses have protein capsids: some also have membrane envelopes around the outside. the shapes of capsids are (almost always) either helical or icosahedral #6) Some virus DNA sometimes gets inserted into the chromosomes of the host (animal or bacteria); stays there for many generations, and then gets cut out again and resumes life as an infective organism. "lysogeny" in bacterial viruses. #7) How do viruses evolve?: two theories; both might be true. a) Highly reduced parasites (further evolution of organisms like Rickettsiae) b) Escaped, run-away genes. like transposons, plasmids #8) Where do new diseases come from (like AIDS) often they have crossed over from some other species. AIDS from Chimpanzees and a certain species of monkey new stains of Influenza: ducks -> pigs -> humans Evolution within one species tends toward doing less harm to host. #9) There are species of virus in bacteria, animals and plants. Any given species typically has hundreds of different viruses, many of which do them very little harm. #10) in sea water, etc. there seem to be many more virus particles than any other (living) things. ------------------------------------ Study Questions 1) Name at least three human diseases that are caused by viruses 2) Which of the following do viruses use to carry their genetic information? double stranded DNA, single stranded DNA, double stranded RNA, single stranded RNA This depends on the kind of virus, and there are many knids of viruses that use double stranded DNA, many other kinds that use single stranded DNA, many other kinds that use double stranded RNA, and others that use single stranded RNA 3) Do antibiotics like penicillin or erythromycin help to cure viral diseases? [hint: no] 4) The layer of proteins that is the outer surface, or part of the outer surface of a virus is called what? [hint: the capsid] 5) What are two very different theories about the origin of viruses? *6) For either or both theories, how would you explain the evolution of those kinds of viruses that use RNA as their genetic material? **7) Might these RNA viruses be related to the original beginning of the first life forms? 8) Do viruses infect animals but not plants; plants but not animals; both plants and animals and also procaryotes? [hint: the last] *9) Only the following regular 3-D shapes are logically possible (in which all the surfaces are regular polygons): a) the cube, whose surface is 6 squares b) the tetrahedron, whose surface is 4 triangles c) the octahedron, whose surface is 8 triangles d) the dodecahedron, whose surface is 12 pentagons e) the icosahedron, whose surface is 20 triangles In a large percentage of viruses, the capsid has the shape of an icosahedron, and no kind of virus has a capsid that is a cube, a tetrahedron, octahedron or dodecahedron. Figure out why. What's the evolutionary advantage to having the shape of an icosahedron? 10) Where did the AIDS virus get started? 11) Do viral base sequences ever get copied into the genome of the host organism? If so, what are these viruses called?

 

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